INF = 1000000

def green(dist, color):
    green_ = -1
    min_ = INF
    for i, t in enumerate(dist):
        if t < min_ and i not in color:
            min_ = t
            green_ = i
    return green_

n = int(input())
d, k = map(int, input().split())
r = int(input())
graph = [[(INF, INF, -1)] for x in range(n)]
for x in range(r):
    s, t1, f, t2 = map(int, input().split())
    graph[s-1].append((t1, t2, f-1))

v = d - 1
dist = [INF] * n
dist[v] = 0
color = {v}

while v != -1:
    for t1, t2, f in graph[v]:
        if t1 != INF and t2 != INF and f != -1 and f not in color and dist[v] <= t1 and t2 < dist[f]:
            dist[f] = t2
    v = green(dist, color)
    color.add(v)

if dist[k-1] == INF:
    print(-1)
else:
    print(dist[k-1])